As described in the 2014 US DOE Report “The Water-Energy Nexus: Challenges and Opportunities”, cooling of thermoelectric power plants accounts for 40% of US freshwater usage and dissipates tremendous quantities (27 quads/year) of primary energy as waste heat due to fundamental inefficiencies in converting thermal energy to electricity; internal combustion engines generate similarly prodigious amounts of waste heat (21.3 quads/year). Devices for mitigating this energy loss employing thermoelectric conversion schemes (i.e. solid-state thermoelectric generators and thermophotovoltaics) exist, but typically require high operating temperatures. An interesting alternative is to exploit the large coefficient of thermal expansion of liquid water to utilize waste heat via the production of high hydrostatic pressures, which can, in turn, drive recently demonstrated electrokinetic energy conversion and hydrogen production processes in fast-flowing liquid microjets.
Electrokinetic conversion of mechanical energy with liquid microjets and nanojets comprises a potentially important, but largely unexplored technology. There have been numerous studies on producing electrokinetic currents by forcing water through both porous materials and/or individual channels. Typically, these experiments generate streaming currents by moving liquid from one reservoir to another through a porous membrane or a single channel by applying pressure to a single side. The direct connection between the two reservoirs introduces significant inefficiencies in the energy conversion due to back-conduction through the liquid. Such inefficiencies are eliminated when liquid jets are employed. Preliminary studies have demonstrated that over 10% of the kinetic energy in a flowing water microjet may be converted into electricity and have shown that the electrical energy production can be accompanied by simultaneous gaseous hydrogen generation. Related U.S. Pat. No. 8,372,374 B2 to Saykally et al, incorporated herein by reference, discloses a device design for converting the kinetic energy in a flowing water microjet into electricity and have shown that the electrical energy production can be accompanied by simultaneous gaseous hydrogen generation. Using a similar system to drive a droplet beam into a region of high electric field, up to 48% efficiency has been reported for in the conversion of the liquid's kinetic energy to potential energy. Additionally, systems and methods are disclosed in a journal article by Lam et al. entitled “Thermally Driven Electrokinetic Energy Conversion with Liquid Water Microjets” incorporated herein by reference in its entirety.
High pressures (>500 PSI) are required to force water through the microjets and generating these pressures may substantially reduce the “wall plug” efliciency. One or more embodiments described herein address this problem by generating such pressures by exploiting water's relatively large thermal expansion coefficient and moderate bulk modulus. For example, by heating the water to very modest temperatures (e.g. 40° C.) in a steel chamber, pressures of about 1200 PSI (about 60 PSI/° C.) may be achieved.